1. Field of the Invention
The present invention relates to a transmitting circuit for use in a portable telephone of the CDMA system or the like.
2. Description of the Related Art
The portable telephone having a transmitting circuit and receiving circuit performs transmission and reception with a base station, whereby the subscribers communicate each other through the base station. Here, since the transmission signal level that the base station transmits to the portable telephones is always constant, the reception signal level that a portable telephone receives varies depending on the distance between the base station and the portable telephone. Further, the base station is designed to receive a signal outputted by a portable telephone always at a constant level.
Therefore, both the receiving circuit and the transmitting circuit of a portable telephone require wide dynamic ranges. Further, in the receiving circuit, the gains of the internal amplifiers are designed to be controlled by the automatic gain control voltage based on the power control signal from the base station, and based on the automatic gain control voltage, the transmission signal level transmitted to the base station from the transmitting circuit is designed to be varied.
The conventional transmitting circuit will be explained with reference to FIG. 4 through FIG. 6. First, in FIG. 4, a QPSK modulated intermediate frequency signal IF of the 200 MHz band is outputted from a modulator not illustrated, and inputted to an intermediate frequency amplifier 21. The intermediate frequency amplifier 21 is configured with a gain controllable variable gain amplifier, and the gain is controlled by an automatic gain control voltage V based on the power control signal from the base station. The automatic gain control voltage V varies from 3.0 volts to 0 volt, and as shown by the curve A in FIG. 5, the gain is controlled to be attenuated from the maximum gain to about xe2x88x9260 dB. The intermediate frequency signal IF is amplified by the intermediate frequency amplifier 21, and thereafter inputted to a frequency converter 22 configured with a mixer 22a and a local oscillator 22b. The intermediate frequency signal IF is mixed in the mixer 22a with a local oscillation signal outputted from the local oscillator 22b, whereby it is converted into a transmission signal RF of about 1.1 GHz band.
The transmission signal RF is first amplified by a high frequency amplifier (called as RF amplifier) 23. The RF amplifier 23 is also configured with a gain controllable variable gain amplifier, whose gain is controlled by the automatic gain control voltage V, and as shown by the curve B in FIG. 5, the gain is made to be attenuated from the maximum gain to about xe2x88x9230 dB. Accordingly, the total attenuation of gain by the intermediate frequency amplifier 21 and the RF amplifier 23 can be secured for 90 dB, as shown by the curve C in FIG. 5. The RF signal amplified by the RF amplifier 23 is further amplified by a driver amplifier 24. The driver amplifier 24 is to amplify the RF signal to such a level as to sufficiently drive a power amplifier 25 at the next stage. The RF signal amplified by the driver amplifier 24 is amplified to a specific transmission level by the power amplifier 25, which is transmitted toward the base station from an antenna 26.
The driver amplifier 24 is configured with a differential amplifier 7 including two transistors 7a, 7b. Both the emitters of the transistors 7a, 7b are grounded through a resistor 7c, and the collectors are supplied with a voltage B through load resistors 7d, 7e. Further, the transmission signal RF amplified by the RF amplifier 23 is inputted to both the bases, and the amplified transmission signal RF is outputted from both the collectors.
In the conventional transmitting circuit thus constructed, the gain of the intermediate frequency amplifier 21 that amplifies the intermediate frequency signal IF of a lower frequency is higher (about double) than the gain of the RF amplifier 23 that amplifies the transmission signal RF of a higher frequency; accordingly to secure the total attenuation of gain, the rate of attenuation gain shared between the intermediate frequency amplifier 21 and the RF amplifier 23 is about two to one at average within the variation range (3.0 volts to 0 volt) of the automatic gain control voltage V. In addition, since the RF amplifier 23 amplifies the higher frequency signal, the gain thereof is saturated as the automatic gain control voltage V becomes high. As a result, in the higher range of the automatic gain control voltage V (for example, from 3.0 volts to 1.5 volts), the rate of the attenuation gain shared with the intermediate frequency amplifier 21 in the total attenuation gain becomes higher (more than 2/3), which deteriorates the SN ratio. Also, in the lower range of the automatic gain control voltage V (for example, from 1.5 volts to 0 volt) that controls the output power into an intermediate power or a low power, the rate of the attenuation gain shared with the intermediate frequency amplifier 21 in the total attenuation gain becomes higher (virtually 2/3), which deteriorates the CN ratio.
On the other hand, the characteristic of the attenuation gain in the receiving circuit is designed to be linear against the automatic gain control voltage. However, as mentioned above, since the RF amplifier 23 is saturated in the higher range of the automatic gain control voltage V, the relation (attenuation gain characteristic) of the automatic gain control voltage V against the attenuation gain does not become linear. In consequence, the characteristic of the total attenuation gain (curve C in FIG. 5) does not become linear, and thereby the matching with the characteristic of the attenuation gain in the receiving circuit cannot be achieved, which is a problem to be solved.
Accordingly, it is an object of the present invention to provide a transmitting circuit that improves the SN ratio in the lower power output (in the higher range of the automatic gain control voltage V (for example, from 3.0 volts to 1.5 volts)) and the CN ration in the lower to intermediate power output (in the lower range of the automatic gain control voltage V (for example, from 1.5 volts to 0 volt)), and makes the attenuation gain characteristic linear.
In order to solve the foregoing problems, the transmitting circuit of the present invention contains an intermediate frequency amplifier that amplifies an intermediate frequency signal, a frequency converter that applies frequency conversion to the intermediate frequency signal into a transmission signal of a higher frequency than the intermediate frequency, a high frequency amplifier that amplifies the transmission signal, and a driver amplifier that further amplifies the transmission signal amplified by the high frequency amplifier and inputs it the result to a power amplifier. And in this construction, the intermediate frequency amplifier, the high frequency amplifier, and the driver amplifier are configured using variable gain amplifiers, and the gain of the intermediate frequency amplifier, the gain of the high frequency amplifier, and the gain of the driver amplifier are made to be varied by an automatic gain control voltage.
Further, in the transmitting circuit of the present invention, the automatic control voltage varies from a first voltage to a second voltage, the gain of the intermediate frequency amplifier and the gain of the high frequency amplifier vary between the first voltage and the second voltage, each of the gains becomes maximum at the first voltage and each becomes minimum at the second voltage, the gain of the driver amplifier is made to be attenuated gradually from the maximum gain to a specific gain, as the automatic gain control voltage varies from the first voltage to a third voltage intervening between the first voltage and the second voltage, and the specific gain is made to be maintained between the third voltage and the second voltage.
Further, in the transmitting circuit of the present invention, the driver amplifier is configured with an amplifier having amplifying elements and a constant current circuit that flows a current through the amplifying elements, the gain of the amplifier is proportional to the current, the current of the constant current circuit is made to be decreased gradually from a maximum current to a specific current, as the automatic gain control voltage varies from the first voltage to the third voltage, and the specific current is made to be maintained between the third voltage and the second voltage.
Furthermore, in the transmitting circuit of the present invention, the constant current circuit is configured with a current mirror circuit including two transistors and a junction FET that controls a current of the transistors, a current running through one of the transistors is made to flow through the amplifying elements, the collector of the other of the transistors is supplied with a fixed voltage through a resistor, through which the collector is also connected to the source of the junction FET, and the gate of the junction FET is supplied with the automatic gain control voltage.